1. Field of Invention
This invention relates to a digital multiplexing system.
2. Description of the Prior Art
A. PULSE STUFFING SYNCHRONIZATION
Digital multiplexing is a concept known in the prior art and has been described, for instance, in paper entitled "Experimental 224 Mb/s digital multiplexer-demultiplexer using pulse stuffing synchronization" by F. J. WITT in the "Bell System Technical Journal," November 1965, pages 1843-1885 and in paper entitled "Digital multiplexing systems" by Yvon MADEC in "Echo des Recherches," January 1973, pages 59-67. Digital multiplexing systems provide multiplexing by the time slotting of component digital signals of one rate into a single resulting digital signal of a higher rate. Digital demultiplexing systems or demultiplexers provide the converse operation. The resulting signal must contain a special recurrent signal which is called the frame locking signal and which gives the demultiplexer a reference.
B. POSITIVE STUFFING MULTIPLEXING OF PLESIOCHRONOUS SIGNALS
In the case of an asynchronous network the clocks of the component signals are independent, but they are plesiochronous, i.e. they all have the same rated frequency and variations therearound are confined within specified limits. Before time multiplexing can be carried out the component signals must be made synchronous with one another. The usual procedure is to equalize the rates upwards by presenting to each component signal a rate slightly above its nominal rate. The difference between the rates is filled up by additional bits called stuffing bits. The multiplexing resulting from stuffing is called positive stuffing multiplexing.
AND DEMULTIPLEXING
The demultiplexer is also required to recognize and remove the stuffing bits associated with each component signal so as to restore it properly. Accordingly, the stuffing bits, if present, have a definite position in the frame (the frame is the interval separated by two consecutive frame locking signals) and their presence or absence is indicated by the logical value of special (systematically inserted) filling bits called "stuffing indication bits."
If N denotes the number of plesiochronous component signals to be multiplexed, F.sub.e denotes their nominal rate and F.sub.s &gt;NF.sub.e denotes the nominal rate of the resulting signal, then: ##EQU1## where Q denotes the number of information bits per frame, P denotes the number of filling bits (frame locking, stuffing indication and, where applicable, service bits) per frame and F.sub.d = .epsilon.F.sub.e denotes the nominal stuffing rate.
c. ASSIGNMENT OF MASTER RATES BY NATIONAL TELECOMMUNICATION AUTHORITIES
A digital multiplexer allots to a number of channels at one rate a single channel at a different rate. The rates of the digital channels are defined by national telecommunication authorities and are called master rates; also, some rates are defined internationally. It may and actually does happen that master rates differ in differents countries. Just as an example, it will be assumed that one country has master rates of 8, 34 and 140 Megabits/second whereas another country has 8 and 140 Megabits/second but not 34 Megabits/second.